1. Field of the Invention
This invention relates to data transmission systems, and more particularly to a digital multiplexing system including a sender for transmitting data pulses to a receiver over a transmission line wherein the data pulses effect synchronization of the receiver with the sender.
2. Description of the Prior Art
Various types of multiplexed data transmission systems have been disclosed in the prior art for transmitting data via a sender from a plurality of locations to a remotely located receiver. In certain systems, special transmission lines are required to permit data transmission between the locations of the sender and the receiver, thus adding to the cost of the system.
In addition, to permit accurate recovery of the data transmitted from the sending location to the receiving location, it is necessary that the receiver be synchronized with the sender. Generally, the sender and receiver each include precision clock circuits, including crystal oscillators, for example. However, the frequency output of the receiver must be set to correspond with that of the sender to permit the receiver to operate in synchronism with the sender. The initial adjustment of the receiver clock during installation may require the use of various measuring instruments which may be undesirable particularly when the receivers are installed at remote locations. In addition, since the sender and receiver clocks operate independently of one another, some provision must be made to compensate for drift of receiver oscillator as may occur due to ambient temperature change or component ageing.
It would be desirable therefore to have an inexpensive multiplexed data transmission system which is capable of monitoring a plurality of points over an extremely long distance over existing communication lines, such as telephone grade lines, for example. It would also be desirable to have a multiplexed data transmission system in which the receiver automatically synchronizes itself with the sender, thereby requiring no special internal clock and eliminating the need for instrumentation to synchronize receiver and sender operations.